1. Field of the Invention
The present invention generally relates to power combiners/splitters in a distributed or coupled line technology. Such devices are used to split an incoming power into two balanced paths or add two incoming powers in a common path. Such devices can generally be found in association with balanced power amplifiers, mixers, phase-shifters, most often to combine several powers obtained from several different amplification paths.
2. Discussion of the Related Art
FIG. 1 is a block diagram illustrating a power combiner/splitter (COMB/DIV) 1. This circuit comprises an access IN, arbitrarily said to be the input access, intended to receive a signal Pin with a power that is to be distributed (or to provide a combined signal), and two accesses OUT1 and OUT2, arbitrarily said to be output accesses, intended to provide distributed power signals Pout1 and Pout2 (or to receive signals with powers to be combined) in phase or in phase quadrature. Not only does circuit 1 have the function of equally distributing power Pin between output accesses Pout1 and Pout2 in phase or in phase quadrature, but also should ensure the isolation between these accesses. Such a device is most often bi-directional, that is, it may be used, according to its assembly in an electronic circuit, to combine two powers Pout1 and Pout2 in a single signal Pin or to equally distribute a power Pin in two powers Pout1 and Pout2.
The present invention more specifically relates to combiners/splitters having their distributed accesses (OUT1 and OUT2) in phase quadrature.
As compared with a coupler having the function of extracting a small part of a power transmitted for measurement purposes, a power combiner/splitter should respect phase imbalance and amplitude imbalance parameters between the distributed paths.
FIG. 2 is a schematic block diagram illustrating a conventional example of a radiofrequency transmission circuit using a combiner (combiner-assembled block 1 of FIG. 1). Combiner 1 is interposed between outputs OUT0 and OUT90 phase-shifted by 90° with respect to each other of two power amplifiers 11 and 12 (PA) of a radiofrequency transmission head 10. Impedance matching circuits 13 and 14 (MATCH), shown in dotted lines, may be interposed between amplifiers 11 and 12 and accesses OUT1 and OUT2 of the combiner. Each amplifier 11, 12 receives a radiofrequency signal RF0, RF90 originating from a phase shift circuit 13 (PHASE SHIFT), which itself receives two differential radiofrequency signals RFin+ and RFin− to be transmitted. Signals RFin+ and RFin− are in phase opposition with respect to each other. Circuit 10 is supplied with a generally D.C. voltage Valim.
Combiner 1 adds signals OUT0 and OUT90 to form a signal IN sent onto an antenna 16 for transmission. A coupler may be added to the combiner to extract data proportional to transmitted power Pout on access IN to possibly adjust the gains of amplifiers 11 and 12.
The same type of architecture may be used for a receive chain. In this case, the combined access (IN) is used as an input terminal while the two distributed accesses (OUT1 and OUT2) are used as phase-shifted output terminals (in phase quadrature) towards two reception inputs of a radiofrequency reception head.
To save the power consumed by the amplification circuits (in transmission or reception), the signals are most often distributed between two paths in phase quadrature. Thereby, the combiners/splitters are generally in phase quadrature for the distributed accesses.
The forming of combiners/splitters may use techniques with lumped elements (association of inductive and capacitive elements) or with distributed or coupled lines (conductive lines arranged sufficiently close to each other to generate an electromagnetic coupling).
FIG. 3 shows a conventional example of a combiner/splitter made in a distributed technology. A first conductive line 21 connects combined access terminal IN to one, OUT1, of the distributed access terminals. A second conductive line, 22, connects a second distributed access terminal OUT2 to a terminal ISO, generally left unconnected. According to whether terminal OUT2 is on the side of terminal IN or on the side of terminal OUT1, the distributed accesses are in phase quadrature or in phase.
In certain cases, terminal ISO is not left unconnected but is loaded with a standardized impedance (typically, 50 ohms). The combiner then becomes directional, that is, a signal entering through terminal IN (antenna 16, FIG. 2) is trapped by terminal ISO to avoid for this signal to reach the application (the amplifiers).
To obtain the combiner/splitter effect, the coupler thus formed should be at 3 dB so that the power of terminal IN is distributed by halves on each of terminals OUT1 and OUT2. In the architecture of FIG. 3, the length of each of lines 21 and 22 should correspond to one quarter of the wavelength (λ/4) of the work frequency of the combiner/splitter, that is, to one quarter of the wavelength of the central frequency of its passband.
A disadvantage of a conventional combiner/splitter such as illustrated in FIG. 3 is its bulk for rather low frequencies, which makes it, in practice, unusable in integrated circuits. For example, for a frequency on the order of one Gigahertz, currently corresponding to the frequencies used in mobile telephony, lines 21 and 22 should exhibit lengths of 34 mm each on a substrate of permittivity ∈r=4.6.
Another disadvantage is that this length of the conductive lines generates high network losses.
It should be noted that a combiner/splitter is fundamentally different from a balun transformer (balun standing for balanced/unbalanced), which comprises one common-mode access and two differential-mode accesses. In particular, a balun does not enable obtaining a quadrature phase-shift, which is used in combiners to which the present invention applies.
Another problem in the forming of a combiner of the type to which the present invention applies is that the coupled lines should be compatible with the currents flowing between amplifiers 11 and 12 and the combiner. Such currents may, in the application to mobile telephony, reach several hundreds of milliamperes. This problem results in significant line widths which adversely affect the miniaturization.